Throttle body gas-flowing - fuelling impact?

[JohnC]

Hi all,

This is probably one for the motorsports fraternity, but who knows? I'd like to know if anyone has gas-flowed the throttle bodies on a TR6 PI (CP specifically, if it makes a difference) and, if so, did it require any adjustment to the fuelling.

The gas-flowing I'm talking about is smoothing of the step left from the original manufacturing, where it appears the bore was machined from each end with different diameters, leaving a step where the two diameters join some distance downstream of the butterflies. For avoidance of doubt, this is nowhere near where the butterfly seats inside the larger diameter.

Thanks,
John

[TriumphV8]

I did so but did not expect anything.

It just looked ugly before .....

But what i did with more hope to do something positive was to equipe the upper 5/16" bolts with a centering that each manifold is fixed in a defined position.

 

Without the throttle plates i fitted the manifolds with the gasket and expanded a flapper grinder with a bolt to equalize the area between manifold and head and established a clean smooth tube.

[JohnC]

Thanks Andreas - I have also port-matched the throttle bodies to the head. In my case I scribed a mark on both to allow precise re-alignment if I disturb them.

[rcreweread]

John - not sure if this is relevant but here goes in case it is.

When I got my first mini at the age of 16, I stripped the engine down and modified and gas flowed the head as per David Vizard and all the advice in the tuning magazine of the day called Triple C ( Cars and Car Conversions) - at the time, I had a holiday job in our local garage .

Having successfully done the head, and now knowing everything there was about gas flowing, I looked inside the inlet tract of my high performance 1 1/4 SU and concluded that SU should have done better, so I ground away the step/ridge in front of the jet to create a lovely smooth inlet tract. 

 

When engine was finished and installed, I road tested the car on my parents drive which was about 50 yards long but I managed to increase this by 5 yards by opening the garage doors at the bottom. 

However the engine just wouldn't run very well at all - once it got revving standing still, it would really rev. Talking to one of the mechanics at the garage about this, he very kindly brought some trade plates along in his lunch hour, and we went for a spin - it was awful and he could see the disappointment on my face, so he set to checking everything - timing. mixture etc and could find nothing wrong.

He then quizzed me as to what I had done to the engine and I explained about gas flowing the head, and then doing the same to the carb, at which point his eyes lit up. He then explained that the bridge was there on purpose to create turbulence in front of the jet which helped to atomise the fuel and as a consequence of what I had done, the carb was now ruined - suitably chastised, a trip to the local scrappy and ten shillings later, I fitted a good second hardhand SU and I was away - engine ran beautifully and when I was old enough to drive and passed my test, it was thrashed to the red line in every gear, every time and never let me down - when I could afford the insurance, I changed it for an MG1100 unit which was even more fun, but that's another story.

So to cut to the chase, it may not be a good idea to remove that casting step but no doubt someone with a lot more experience of PI throttle bodies will be along to advise

Cheers Rich

[JohnC]

Thanks Rich. Fortunately the step in the throttle body is downstream of the injector, but the same may still apply. The advice to blend it came from a machine shop that does gas-flowing for competition TRs here in Sydney, so hopefully it's good advice. But, as I'm often reminded, an engine is a system that needs to work in harmony...

My enquiry was prompted by the tuning work I'm currently doing after the gas-flowing of head and TBs, and swapping the cam (to a "150bhp" CP-profile). I've set the MU to exactly match the TR5 fuel delivery curve, but I'm getting a backfire when going suddenly to WOT from a trailing throttle. I can also stimulate it by WOT acceleration to the red line+ followed by a fast gear change & WOT again. Richening the min fuel stop solves the problem, at the expense of a very rich idle. I'm pretty sure this is an illustration of the "lean spike" @Peter Cobboldand others have mentioned.

I'm going to carry on experimenting with fuel delivery curve but thought it worth checking in with the knowledge bank!

Cheers,
John

[Nigel Triumph]

1 hour ago, rcreweread said:

John - not sure if this is relevant but here goes in case it is.

When I got my first mini at the age of 16, I stripped the engine down and modified and gas flowed the head as per David Vizard and all the advice in the tuning magazine of the day called Triple C ( Cars and Car Conversions) - at the time, I had a holiday job in our local garage .

Having successfully done the head, and now knowing everything there was about gas flowing, I looked inside the inlet tract of my high performance 1 1/4 SU and concluded that SU should have done better, so I ground away the step/ridge in front of the jet to create a lovely smooth inlet tract. 

 

When engine was finished and installed, I road tested the car on my parents drive which was about 50 yards long but I managed to increase this by 5 yards by opening the garage doors at the bottom. 

However the engine just wouldn't run very well at all - once it got revving standing still, it would really rev. Talking to one of the mechanics at the garage about this, he very kindly brought some trade plates along in his lunch hour, and we went for a spin - it was awful and he could see the disappointment on my face, so he set to checking everything - timing. mixture etc and could find nothing wrong.

He then quizzed me as to what I had done to the engine and I explained about gas flowing the head, and then doing the same to the carb, at which point his eyes lit up. He then explained that the bridge was there on purpose to create turbulence in front of the jet which helped to atomise the fuel and as a consequence of what I had done, the carb was now ruined - suitably chastised, a trip to the local scrappy and ten shillings later, I fitted a good second hardhand SU and I was away - engine ran beautifully and when I was old enough to drive and passed my test, it was thrashed to the red line in every gear, every time and never let me down - when I could afford the insurance, I changed it for an MG1100 unit which was even more fun, but that's another story.

So to cut to the chase, it may not be a good idea to remove that casting step but no doubt someone with a lot more experience of PI throttle bodies will be along to advise

Cheers Rich

Advance apologies for slight thread drift...

I love Rich's story of the tuned Mini, having also fitted an MG1100 engine to a Mini as a teenager, when Triple C was the must have car magazine. I rebuilt the engine, including gas flowing the head per David Vizard and fitting a 997 Cooper camshaft. Fuelling was via a pair of H4 1.5" SUs and the exhaust was big bore with a long centre branch manifold.

Eighteen year old me thought it was the dog's whatsits! The car could hold 1275 Cooper S away from the lights. Then I saw the light and bought my first Triumph, a Spit Mk3, and I've owned Triumphs almost ever since.

Nigel

[harrytr5]

And there is me thinking I was the only one in fitting a MG 1100 engine with twin choke Nikki carb. 6" steering wheel fibreglass hinged front. Dunlop alloys. Used to get wheel spin in third or was it 2nd gear. It sure did fly. Happy days.

Regards Harry.

[JohnC]

49 minutes ago, harrytr5 said:

And there is me thinking I was the only one in fitting a MG 1100 engine with twin choke Nikki carb. 6" steering wheel fibreglass hinged front. Dunlop alloys. Used to get wheel spin in third or was it 2nd gear. It sure did fly. Happy days.

Regards Harry.

You weren't by chance at Uni of Southampton in the early '80s? I remember one of those...

[harrytr5]

No not I John. Not intelligent enough for Uni.

Regards Harry